Understanding Auroras: Science, Colors, and Northern Lights

How Auroras Work

Introduction to How Auroras Work

If you're camping near the United States/Canada border or points farther north, you might see an eerie glow in the night sky. Sometimes it can look like twilight. At other times it can look like a glowing, dancing ribbon of light. The light may be green, red, blue or a combination of these colors. What you are seeing is called the aurora borealis, or simply an aurora.

Auroras have signified different things to different cultures. The Vikings thought that auroras were reflections off the armor of the mythical Valkyries. To the native Eskimos of Greenland and nearby Canada, auroras were communications from the dead. To American Indians, they were lights from huge campfires far ­to the north. In medieval times, auroras were omens of war or disasters, such as plague. Today, we know that they're a light phenomenon caused by high-energy particles from the sun's solar winds interacting with the Earth's magnetic field. However, knowing the physical reason for auroras certainly doesn't detract from these beautiful natural light shows.

Because auroras are caused by the interaction of solar winds with the Earth's magnetic field, you can see them most often near the poles, both north and south. In the north, they're called aurora borealis, or Northern Lights. Aurora is the name of the Roman goddess of the dawn, and "boreal" means "north" in Latin. In the southern hemisphere, auroras are called aurora australis (Latin for "south").

Auroras follow solar cycles and tend to be more frequent in the late autumn and early spring (October, February and March are the best months for seeing them). Around the Arctic Circle in northern Norway and Alaska, you can see them almost nightly. As you travel south, their frequency diminishes. Around southern Alaska, southern Norway, Scotland and the United Kingdom, they might show up about one to 10 times per month. Near the United States/Canada border, you may see them two to four times a year. Once or twice a century, they might pop up in the southern United States, Mexico and the equatorial regions.

Let's take a closer look at auroras and what causes them.

Launch Video The Beauty of Auroras

What do auroras look like?

As we mentioned, auroras take on different appearances. They can look like an orange or red glow on the horizon -- like a sunrise or sunset. Sometimes they may be mistaken for fires in the distance, like the American Indians thought. They can look like curtains or ribbons and move and undulate during the night.

Auroras can be green, red or blue. Often they will be a combination of colors, with each color visible at a different altitude in the atmosphere.

• Blue and violet: less than 120 kilometers (72 miles)
• Green: 120 to 180 km (72 to 108 miles)
• Red: more than 180 km (108 miles)

After a particularly active solar maximum in the sun's cycle, the red color may appear at altitudes between 90 and 100 km (54 to 60 miles).

Oxygen ions radiate red and yellow light. Nitrogen ions radiate red, blue and violet light. We see green in regions of the atmosphere where both oxygen and nitrogen are present. We see different colors at different altitudes because the relative concentration of oxygen to nitrogen in the atmosphere changes with altitude.

Auroras can vary in brightness. People who regularly observe auroras and report on them generally use a rating scale from zero (faint) to four (very bright). They'll note the aurora's time, date, latitude and colors and make quick sketches of the aurora against the sky. Such reports help astronomers, astrophysicists and Earth scientists monitor auroral activities. Auroras can help us understand the Earth's magnetic field and how it changes over time.

Because the Earth's magnetic field is three-dimensional, the aurora appears as an oval ring around the pole. This has been observed from satellites, the International Space Station and the space shuttle. It isn't a perfect circle because the Earth's magnetic field is distorted by the solar winds.

The auroral ring can vary in diameter. Auroras can be seen as far south as the southern United States, but not frequently. In general, they stay near the polar regions. They also occur in pairs -- when we see an aurora borealis, there is a corresponding aurora australis in the southern hemisphere (learn why on the next page).

Do auroras occur only on Earth?

Because auroras are caused by the interactions of solar winds and solar flares with the magnetic fields of a planet, you'd think they'd happen on other planets as well. What you need is:

• Solar flares and winds that provide the charged particles and energy to interact with a planet’s magnetic field
• A planetary magnetic field (probably of some strength) that traps electrons from space
• A planetary atmosphere that contains ionic gases that interact with energetic electrons from the magnetic field and produce light through excitation and relaxation of their electrons

So, with these conditions, we have observed auroras on Jupiter and Saturn. Both planets have powerful magnetic fields and atmospheres with ionized gases, mainly hydrogen and helium.

The Hubble Space Telescope caught images of auroras on Jupiter, and the Cassini probe orbiting Saturn has photographed auroras there. ­

What causes auroras?

Auroras are indicators of the connection between the Earth and the sun. The frequency of auroras correlates to the frequency of solar activity and the sun's 11-year cycle of activity.

As the process of fusion occurs inside the sun, it spews high-energy particles (ions, electrons, protons, neutrinos) and radiation in the solar wind. When the sun's activity is high, you'll also see large eruptions called solar flares and coronal mass ejections. These high-energy particles and radiations get released into space and travel throughout the solar system. When they hit the Earth, they encounter its magnetic field.

The poles of the Earth's magnetic field lie near, but not exactly on, its geographic poles (where the planet spins on its axis). Scientists believe that the Earth's liquid iron outer core spins and makes the magnetic field. The field is distorted by the solar wind, getting compressed on the side facing the sun (bow shock) and drawn out on the opposite side (magnetotail). The solar winds create an opening in the magnetic field at the polar cusps. Polar cusps are found on the solar side of the magnetosphere (the area around the Earth that's influenced by the magnetic field). Let's look at how this leads to an aurora.

1. As the charged particles of solar winds and flares hit the Earth's magnetic field, they travel along the field lines.
2. Some particles get deflected around the Earth, while others interact with the magnetic field lines, causing currents of charged particles within the magnetic fields to travel toward both poles -- this is why there are simultaneous auroras in both hemispheres. (These currents are called Birkeland currents after Kristian Birkeland, the Norwegian physicist who discovered them -- see sidebar.)
3. When an electric charge cuts across a magnetic field it generates an electric current (see How Electricity Works). As these currents descend into the atmosphere along the field lines, they pick up more energy.
4. When they hit the ionosphere region of the Earth's upper atmosphere, they collide with ions of oxygen and nitrogen.
5. The particles impact the oxygen and nitrogen ions and transfer their energy to these ions.
6. The absorption of energy by oxygen and nitrogen ions causes electrons within them to become "excited" and move from low-energy to high-energy orbitals (see How Atoms Work).
7. When the excited ions relax, the electrons in the oxygen and nitrogen atoms return to their original orbitals. In the process, they re-radiate the energy in the form of light. This light makes up the aurora, and the different colors come from light radiated from different ions.

Note: The particles that interact with the oxygen and nitrogen ions in the atmosphere don't come from the sun, but rather were already trapped by the Earth's magnetic field. The solar winds and flares perturb the magnetic field and set these particles within the magnetosphere in motion.

For more information on auroras, take a look at the links on the next page.

How do we know what causes auroras?

In 1895, a Norwegian physicist named Kristian Birkeland addressed the queston of what causes auroras. Birkeland believed that auroras were caused by electrons from the sun that interacted with the Earth's magnetic field. To test this, he placed a spherical magnet called a terrella inside a vacuum chamber. He also had an electron gun inside the chamber. When he turned on the gun, electrons interacted with the magnet's field and produced an artificial aurora, supporting his hypothesis.

Birkeland's artificial aurora didn't show the characteristic oval ring. The auroral ring was actually predicted by a Japanese graduate student named Shun-ichi Akasofu in 1964. He examined photographs of auroras and concluded that auroras were rings. So, why weren't Birkeland's auroras oval? Birkeland thought the electrons that excited the oxygen and nitrogen ions came directly from the sun. Only when satellites began to study auroras and measure the magnetosphere did scientists figure out that the electrons came from the magnetosphere itself. When this idea was placed in mathematical models, auroral rings could be explained.